It means the ECU has logged a fault — anything from a loose fuel cap to a failing sensor. A diagnostic scan reads the code and tells you the cause.
Here's a quick, honest answer from a working UK mechanic — what causes it, what it costs to fix, and what to do next.
Will my car fail an MOT with the EML on? — Yes. A permanently illuminated MIL/EML is an automatic MOT failure on any vehicle from March 2014 onwards.
Can I clear the light myself? — You can with a code reader, but unless the underlying fault is fixed it will return — usually within a few drive cycles.